(core) Billing for formula assistant

Summary:
Adding limits for AI calls and connecting those limits with a Stripe Account.

- New table in homedb called `limits`
- All calls to the AI are not routed through DocApi and measured.
- All products now contain a special key `assistantLimit`, with a default value 0
- Limit is reset every time the subscription has changed its period
- The billing page is updated with two new options that describe the AI plan
- There is a new popup that allows the user to upgrade to a higher plan
- Tiers are read directly from the Stripe product with a volume pricing model

Test Plan: Updated and added

Reviewers: georgegevoian, paulfitz

Reviewed By: georgegevoian

Subscribers: dsagal

Differential Revision: https://phab.getgrist.com/D3907

app/client/components/DocComm.ts

@@ -32,7 +32,6 @@ export class DocComm extends Disposable implements ActiveDocAPI {
   public addAttachments = this._wrapMethod("addAttachments");
   public findColFromValues = this._wrapMethod("findColFromValues");
   public getFormulaError = this._wrapMethod("getFormulaError");
-  public getAssistance = this._wrapMethod("getAssistance");
   public fetchURL = this._wrapMethod("fetchURL");
   public autocomplete = this._wrapMethod("autocomplete");
   public removeInstanceFromDoc = this._wrapMethod("removeInstanceFromDoc");